The amplitude of the output of a signal generator may vary when the specified frequency is changed, and the output may change non-linearly when the specified amplitude is changed; these are undesirable effects. If the amplitude of such a signal generator is to be maintained constant, corrections must be made for both effects.
The frequency effect is a variation of output amplitude as a function of frequency, which is often a result of voltage-standing-wave ratios (VSWR), etc. The amplitude effect is a variation of achieved output amplitude as a function of specified amplitude, which is due to non-linearity of the relationship between specified amplitude and achieved output amplitude over the available amplitude range.
One prior method of making the necessary corrections involves the use of software or firmware. When a user specifies a given output amplitude and frequency, a control component (such as a microprocessor) of the prior system makes the necessary corrections under the control of a clock-paced program in a central processing unit (CPU). The CPU first looks up the amplitude correction in a correction table, based on the specified frequency value (and upon the specified amplitude value). This yields the amplitude-vs-frequency correction, which is a signed value. Then a second value is read by the microprocessor from a second calibration table, based upon the specified amplitude. This value is added by the microprocessor to the first and is used to correct the output amplitude.
An important limitation of the prior method is that it takes a long time for the microprocessor of the signal generator to perform the amplitude corrections. This limits the use of the amplitude-correction feature to the fixed-frequency mode of operation of the signal generator, i.e., it prevents the use of the correction feature during a swept-frequency mode of operation. Moreover, even in the fixed-frequency mode the correction does not settle to steady state until after the frequency has stabilized, a fact that limits the switching speed of the signal generator of the prior system.